A systematic study of cross-talk limitations in RPC timing

A systematic study of cross-talk limitations in

RPC timingD. Gonzalez-Diaz, A. Berezutsky and M. Ciobanu

for the CBM-TOF working group

15-10-2008

Index

1. Cross-talk and timing. General remarks.

2. Measurements and comparison with FEM (Finite Element Method) description.

3. Design studies for operation in differential mode.

4. Conclusions.

Basics (RPC rise-time)

D.Gonzalez-Diaz, PhD(2006)

measured with UV source and double-threshold technique

tSoeii avalanche growth until onset of Space-

Charge

P. Fonte et al., IEEE, Trans. Nucl. Sci. 49,3(2002)881

if Space-Charge present already at the level of the comparator, S would beeven higher !

also

Basics (RPC rise-time)

ps2009ln

S(E)

)(trise E=100kV/cm

time domain

GHzt

frise

c 75.135.0

frequency domaincmfc cc 17

FT

(V)

Basics (Q distribution and usual threshold)

HADES 2003 prototype

D.Gonzalez-Diaz, PhD(2006)%10/ signalthreshold VV

2.1/

/

promptq

signalV

qrms

Vrmssignal

not including streamers

measured directly in the scope

Basics (fluctuations in time response)

2talkcross

2noiseexternal

2noiseintrinsic

2propag

2RPC

2T,total

independent sources!

2talkcross

2event single

2T,total

We believe we can make

σsing;e event~80 ps for large

systems (CBM-Techincal Status Report 2005)

?

Cross-talk influence in timing (simple

derivation)

log(Vsignal)

t

Vth

variations in base-line due to cross-talk

variations in time at threshold due to cross-talk2

talkcross

space-charge

exponential regime


derivation)Assumptions: Within the same primary collision cross-talk extends up-to infinite time. It does not depend on position. Fluctuations in time of cross-talk signal are smaller than fluctuations coming from the avalanche charge distribution. Pick-up strips are separated by a typical distance bigger than the area of influence of the avalanche. Charge sharing during induction can be neglected!. Cross-talk influences only the first neighbour, coupling to it with a fraction of its amplitude F. Cross-talk is small.

S

F

V

rms

V

Vrms

signal

V

threshold

signal signal

talkcross


derivation)

eventsingletalkcross rmsrms

SrmsV

rms

V

VF

signal

V

signal

threshold )( eventsingle

1

psrms 80event-single 1.0F


derivation)

psrms

psrms

MCtalkcross

analyticaltalkcross

87

60

events above threshold in neighbour: 20%

cross-talk: F=10%

subtle cross-talk(below threshold)

rough cross-talk(above threshold)

Measurements of cross-talk with RPC mockup

Different ways of shielding

shielding viasshielding strips

Comparison with

simulationsignal injected from a fast scintillator trise~1ns

Cross-talk from cell without shielding

Cross-talk from cell with shielding strips

Cross-talk from cell with shielding vias

Cross-talk from cell with shielding vias(neighbour not terminated on the other side!)

Cross-talk from cell with shielding vias(to 1st and 2nd neighbour!)

simulation of the S coefficient

scattering matrix coefficient to neighbouring anode (equivalently: fraction of signal transmitted)

Comparison with data from spectrum analyzer

preliminary!

simulation of a realistic structure

propagation of exponential signal with 200 ps rise-time in anode and cathode simultaneously (differential mode)

RPC structure: signal width = 22 cm, gap to next strip = 0.3 cm

16 gaps,0.16 mm gap0.3 mm glass0.86 mm PCB

Transmission properties (with vias)








Transmission properties

vias

no vias


vias

no vias


vias

no vias


vias

no vias


vias

no vias


vias

no vias


vias

no vias


vias

no vias

all in a nut-shell

Dependence with strip length




Conclusions (I)

Cross-talk levels at the level of 10% are capable of destroying timing RPC multi-hit performances.

Cross-talk influence depends critically on the ratio Vth/<Vsignal> and the

signal rise-time. For timing RPCs it corresponds to a 'corner frequency' of 1.75 GHz at least (-3dB drop).

Aplac is a good tool for understanding fast signal propagation. For the measured signals the quality of our groundings is well described

by 'ideal grounding' in APLAC. Detector box affects critically to signal properties. Without a proper

description the simulation fails by factors.

Outlook

Elaborate the results a bit further to establish sound requirements. Use this information to build a prototype. Measure cross-talk in a realistic prototype. Measure experimentally the charge-sharing during induction.

Conclusions (II)

Impedance matching does not help at all from the point of view of cross-talk, however it may allow for desired double-hit capabilities.

Detector matching is extremely complicated and does not fit at all to the required granularities. If impedance matching is requested, the granularity at the outer regions would require strips of 4 mm wide and 6 meters long.

How to get a matched detector?? (first of all, the resulting multi-strip configuration is only truly matched after ALL the corresponding cross-impedances are matched):

Build the detector with the impedance of the FEE. Build the FEE and cables with the impedance of the detector. Build the detector and the FEE and match the impedance with some

driver. Do not care about double-hit capabilities in the same strip (HADES).

Conclusions (III)

suggestion: place the FEE inside the detector,

reduce its input impedance and eliminate

the problem of signal transmission in cables!

Documents

A systematic study of cross-talk limitations in RPC timing